Over a decade, coal-based thermal power plants are upgraded to operate at supercritical pressure conditions due to its high efficiency and low emissions. Water wall panels of a typical supercritical boiler are structured spirally in the lower furnace and vertically placed in the upper furnace. The spiral tubes are inclined at 19 to 22 degrees in which fluid behaves as in horizontal tubes. The design of water wall panels plays the key role in designing a supercritical boiler. The present work aims to numerically investigate the heat transfer behavior of both vertical and horizontal tubes at the supercritical conditions. Since the temperature distribution across the cross-section of vertical tube is uniform, a 2D axis symmetry tube has been considered for analyzing the vertical tube. Unlike vertical tube, the heat transfer characteristics is different for horizontal tubes. Therefore, a 3D tube has been modelled for the computation of horizontal tubes. In order to gain confidence, the present simulations are validated with experiments results available in the literature. Ansys-Fluent has been used in the present simulation. SST k-ω turbulence model is used in this analysis. In the present work, 10 mm diameter of 4m length of vertical tube has been chosen and simulated at low heat flux to mass flux ratio 0.27 and high heat flux to mass flux ratio 0.67 with pressure 241 bar. The effect of heat flux (q) to mass flux (G) ratio which is responsible for heat transfer enhancement and heat transfer deterioration has been studied for both vertical and horizontal tubes. The wall temperature has been plotted along the length of the tube for both top and bottom portion of horizontal tube and compared with wall temperature of vertical tube. The effect of buoyancy plays a vital role in the heat transfer behavior of horizontal tube compared to vertical tube. Heat transfer deterioration occurs due to buoyancy which has a direct linkage with gravity. Three cases were studied, one with full gravity (factor 1), half gravity (factor 0.5) and zero gravity (factor 0). It has been observed that, sudden rise in wall temperature occurs for the case gravity factor 1.0, i.e, considering the gravity effect. For the case of zero gravity, no sudden peak of local wall temperature is observed due to the absence of buoyancy term in the Navier-Stokes equations. Some of the thermo-physical properties like velocity, turbulent kinetic energy, density, wall temperature and turbulent viscosity are analyzed for three cases.